When you think about science fiction theme tunes, chances are there are a few that are especially stirring and heroic. Star Wars. 2001: A Space Odyssey. Star Trek: The Next Generation. Superman: The Movie. And all of these theme tunes have something in common: they rely on the same basic intervals.

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We talked to music experts — including legendary composer Bear McCreary — to find out why so many famous theme tunes use the "perfect fifth" for their hook.

Most people will instantly recognize the first few notes in 2001: A Space Odyssey, which was originally known as "Also sprach Zarathustra" by Richard Strauss. It starts with a low C, and then goes up five notes to a G — that's a perfect fifth right there. And then the next note is another C, up an octave from the first C.

But the Star Wars theme, by John Williams, relies on a similar progression. The first few sustained notes in Star Wars are a G, going up a perfect fifth to a D, and then a higher G. Williams also plays with a descending perfect fifth in the Superman: the Movie score. And his E.T.: The Extraterrestrial theme also starts with an ascending perfect fifth.

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You'll find the "perfect fifth" prominently represented in many other classic theme tunes, including the original Battlestar Galactica. And if you pick out just the sustained notes in Jerry Goldsmith's Star Trek: The Motion Picture score, which became the theme from Star Trek: The Next Generation, you find a perfect fifth, followed by the octave.

What's so special about a perfect fifth?

We were lucky enough to spend 15 minutes on the phone with McCreary, the composer whose music helped to define the Battlestar Galactica reboot as well as Terminator: The Sarah Connor Chronicles and many other shows. He told us:

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You're talking about two intervals: a perfect fifth and [then] a perfect fourth. For example, if you're starting at middle C, you go up a perfect fifth, up to G. You go up a fourth, and you're back to C. This is the core relationship in Western tonal music, which is 99.999 percent of all music that is used in scores for anything that Western audiences generally watch. It's the basis of all pop music and Western classical music.

The question, says McCreary, is: "Why is this set of intervals so commonly used, and so powerful and so effective?"

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Where did the iconic perfect fifth come from?

"It has its basis in physics," says McCreary. "It's a physical reality." There's an actual physical phenomenon behind the perfect fifth, and the octave above that, called the "overtone series." Here's how it works, according to McCreary:

When you hit a string or a piece of metal or anything that vibrates, you hear the fundamental pitch that the thing is vibrating at, but you hear a series of overtones - of harmonic frequencies - that you're not exactly aware that you're hearing. And those are a series of notes that are increasingly higher than the fundamental. So what's called the first overtone, as in the lowest of the overtones, is an octave higher than the fundamental, and you can guess what the next one up is: It's a fifth higher up than that.

So if you have a guitar string that is tuned to a C and you pluck it, you actually hear not only that C, [but also] you hear clearly the C above that, and less clearly the G above that. And in fact, you're hearing many, many more notes that keep going up higher and higher, but the higher you go the less clear it is. There is something fundamentally natural about that octave, and then a fifth relationship, that happens in sound. This is not something that composers came up with this, this is something that happens in the physical universe as we know it. So that makes it feel very strong... because you play a note and then you play the octave above it, you're reinforcing overtones.

In other words, when you play a C and then a G, and then a higher C, audiences will also be hearing the overtones of the higher C and G above those notes. The notes will end up feeding back on themselves, says McCreary. This gives a feeling of strength, because "it's reinforcing itself in a very powerful way. So if you want to indicate something that has strength or grandeur, this is a really simple and powerful way of doing that. You're building these notes that have versions of themselves built into themselves."

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These are the notes we're used to hearing all the time, says McCreary: "If you take a pan out of the oven and smack it, you're going to hear those same notes resonating out. And I think that same familiarity makes it comforting."

And besides Strauss' "Zarathustra," some other iconic themes use this interval – for example, there's Aaron Copland's "Fanfare for the Common Man," points out Jeremy Barham, a senior lecturer in music at the University of Surrey. This piece is "pervaded by intervals of fourths, fifths and octaves, [and] has inspired film composers searching to express the fearlesss, pioneering spirit of exploration."

And both Strauss and Copland were trying to use these intervals to express something larger and more impressive, says Neil Lerner, a professor music at Davidson College who's written about science fiction theme music in books like Off the Planet: Sound and Science Fiction Cinema:

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Strauss is doing it for some reasons connected to his extramusical program: the opening part of the tone poem is what he calls his Nature theme, and it seems to be representing the part of the Nietzsche where Zarathustra comes out and sees a sunrise in the world and thinks about nature. ... That openness [of the overtone series] can register as being open opportunities or a big space or all sorts of other things. When Copland does it in something like Appalachian Spring, it's connected to the idea of the physical geography of open prairies but also... to the notion of the American dream/myth of limitless opportunity. Hence, Copland's style gets adopted into genres like the Western but also into science fiction.

But even before 2001 and Star Wars, composers were playing with these intervals in movie scores, notes Lerner — just check out the swashbuckling scores by Erich Wolfgang Korngold, especially Kings Row. "The typical reaction people have when they first hear the Kings Row theme is that John Williams is a huge plagiarist," says Lerner. But that's unfair, because Williams was clearly trying to pay homage to the 1930s and 1940s swashbuckling movie scores, and he puts his own stamp on them.

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Why do science fiction composers love it so much?

It really does come down to Stanley Kubrick's choice of Richard Strauss' "Zarathustra" for 2001: A Space Odyssey, according to McCreary. This choice "sent a very powerful signal, a connection between open fifths and fourth, and in particular with brass implementation."

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Barham adds that Kubrick's music choice "was an extremely important catalyst for subsequent musical characterizations of the cosmos, or transcendence or generally mind-blowing scenarios in film." That series of ascending intervals "acts as an iconic symbol of things that are uplifting, progressive, bold, active - all heroic traits associated with space explorers and adventures."

And then came Star Wars, and McCreary says we can assume that John Williams, "like everyone else, was aware of 2001, and that that was something that he and Lucas wanted to draw upon. That kind of sound. Obviously you'd have to ask one of them to know for sure, but I think it's a safe presumption."

Before 2001 and Star Wars, says Barham, science fiction movie scores were much less reliant on the perfect fifth and big brassy scores. If you listen to scores from movies from the 1950s and 1960s, like Forbidden Planet or The Day The Earth Stood Still, "some very different things are going on."

And then came Jerry Goldsmith's Star Trek theme, in 1979. McCreary calls it "one of the great theme songs." And even though it totally relies on the same intervals as Star Wars and 2001 in its "brassy fanfare," it also has "a lot more interesting melodic and harmonic content than, say, the Superman theme."

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"It's hard to take a series of intervals that we all recognize, and arrange them in a way that is not so simplistic, to give the audience that's a little bit of a challenge but still retains that catchiness," says McCreary. "Goldsmith was the master of that."

How Bear McCreary has used the perfect fifth — or chosen not to

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Listen to McCreary's theme tune to The Cape, and you'll hear a descending perfect fifth over and over again. McCreary did this on purpose, to invoke the great superhero theme tunes of all time. "When you look at the theme to The Cape, you're going to notice that I take one structure and repeat it up a minor third. So you hear that descending fifth, and then I actually have another one."

This took a lot of thought on McCreary's thought, because he wanted to "tip my hat to the great genre writers of the past," while tweaking it in a way that made it more personal:

Because that open fifth is so well known, and it sounds like Star Wars, and it sounds like 2001, I was trying to find my own twist on it. And that was my solution to it: not only having one set of an open fifth, but having another one that was up a minor third. So it almost feels like the theme to the Cape is in two keys.

He does something similar with his theme tune to Human Target as well, which features a similar leap. "When I wrote that, I was almost nervous that you can't do that any more, because it's been done so many times. But ultimately, when you're trying to write music, particularly if you're doing music in the Western style, you only have 12 notes to choose from." So it's not about whether you use these intervals, but how you make them your own.

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Ultimately, says McCreary, a theme song is supposed to be memorable. "If it's not memorable, it's not doing its job."

But sometimes, it's more effective to avoid hitting those intervals too hard — take the theme from The Simpsons by Danny Elfman. It's a fun, comical tune that sticks in your head and sounds kind of... wrong. This is because Elfman "took the open fourth and fifth and went for the tritone in between," says McCreary. "Everything about the Simpsons arrangement is fun and it's zany, but... it sounds like music we've heard before. But that tritone in there makes it feel like there's something wrong. Right away, I feels like there's something not quite right. And I think he went out of his way to avoid exactly these open fifths and open fourths that we're so used to hearing."

And of course, McCreary's own score for Battlestar Galactica avoids those heroic intervals, which are so prominent in the original Battlestar theme tune. "This was a direct response to the original Battlestar [theme] by Stu Phillips," as well as those other famous like Star Wars and Star Trek: TNG, says McCreary. "It's hard to remember now, because the show has become so lauded, but at the time nobody wanted to see a new Battlestar Galactica." So it was a way of sending "sort of a daring message," choosing not to do "a science fiction score with that same sort of brassy approach."

But even though you don't associate the BSG reboot with that same thundering brass, full of strong fifths and fourths, McCreary confesses that "I still used all the same tricks." In particular, the "Lords of Kobol" theme which was introduced towards the end of season one "starts off with a giant obvious open fifth, and I did that so that it would be memorable." But of course, because it's played on an Asian musical instrument or sung by a single ethereal voice, it doesn't sound at all the same as the theme from 2001.